You know it's a "murder of crows" and a "wake of buzzards" but it's a what of ravens, again?
The answer is an "unkindness of ravens". Likewise, it's a "dole of doves", a "charm of finches", a "scold of jays", a "deceit of lapwings", a "watch of nightingales", a "parliament of owls", and an "exaltation of larks".
Or so they say. It's an interesting convention in English, giving a morally symbolic name to a flock of birds or a herd of beasts. Some of these, like a dole (or dule) of doves, go back quite far -- this use of 'dole' or 'dule' goes back at least to the fifteenth century Book of St. Albans; 'dole', of course, means sadness, as in the mourning coo doves make. This is also where we get "unkindness " as the name for a flock of ravens (probably reference to folklore about how ravens treat their young). Book of St. Albans is the most common name of a work often attributed to Dame Juliana Berners on subjects like hunting and heraldry. In the mix there is a list of "companies of beasts and fowls" (which you can find here by going to page 114); the names it attributes to these collectives are 'terms of venery', that is, they are hunters' terms. But even in the fifteenth century, Dame Juliana was having a bit of fun with the naming, since among these various collective nouns for animals we find entries like "superfluity of nuns", "bevy of ladies", "disguising of tailors", and "pontificality of prelates", as well as, if I am reading it correctly, "incredibility of cuckolds" and "abominable sight of monks". So we don't have any way of knowing which of the animal ones were really used by hunters and which of them is the author joking. The jokes seem to have been quite widespread, though, since there are other lists of this sort from around the same time. The list that we get printed by Wynken de Worde is very similar, for instance, to a list given with Caxton's printing of John Lydgate's The Debate of Horse, Goose, and Sheep, but the lists from the two printers are not exactly the same; I think a lot of popular references do not clearly distinguish the two lists, and I don't have the kind of access to manuscripts and early editions that would allow me to untangle this.
The list made it into Gervase Markham's The Gentleman's Academy (1595), which is basically a revision of the Book of St. Albans; Markham was an extraordinarily popular author in his day, and thus the convention was established. A lot of the collective nouns for animals and birds go back to this one list, including "muster of peacocks", "barren of mules","gaggle of geese", and "pride of lions". That's where English gets its double tradition of collective nouns -- we have a generic version ("flock" or "herd") and for animals that are well known we have a venereal or fanciful version.
For all fanciful collectives that do not go back to these original sources, and for some of those that do, the history of the term is very difficult to trace. "Pride of lions" and "gaggle of geese", which are probably the most widely used fanciful collectives, are found in the original lists and then are hardly heard of until a resurgence in the nineteenth century; I suspect in part due to Joseph Strutt's The Sports and Pastimes of England (1801), one of the places the old list resurfaces. "Murder of crows" seems to be from the Lydgate (Caxton) list. In any case, its resurgence is more recent, through James Lipton's An Exaltation of Larks (1968), which is responsible for the resurgence of a number of the original terms.
But the tradition seems to have become self-sustaining; people keep adding to the list from all sides, often with very bad puns. Some of these can't be traced, but some novel additions have an entry into English that can be pinpointed exactly. For instance, "surprise of unicorns"; we learned that this was the company term for unicorns in Jane Yolin's "The Boy Who Drew Unicorns" in the 1988 Doubleday book, Unicorn Treasury: Stories, Poems, and Unicorn Lore. "Flight of dragons", as far as I can determine, comes from the 1982 TV movie, The Flight of Dragons, and its theme song, by none other than Don McLean.
So that's more or less the story, as far as I can tell with my limited means for researching it, of one of the English language's more charming and playful features, the venereal collective or fanciful company term.